Field of the Invention
The present invention relates to a technique for detecting overlapping of a plurality of sheets with a sheet conveyance apparatus or the like.
Description of the Related Art
A sheet conveyance apparatus for conveying sheets is used in image forming apparatuses and image reading apparatuses. It is presumed that sheets are conveyed one by one (which will be called “single-feed”), and conveyance of the sheets stops if a plurality of sheets are conveyed in an overlapping manner (which will be called “double-feed”). Japanese Patent Laid-Open No. 2012-188177 proposes an ultrasonic sensor for detecting double-feed. In particular, Japanese Patent Laid-Open No. 2012-188177 proposes changing the degree of amplification (gain) of a received signal in accordance with sensitivity variation in the ultrasonic sensor.
Incidentally, a signal that is output by an ultrasonic sensor is amplified by an amplifying circuit, but the detection accuracy may decrease due to noise components such as exogenous noise and semiconductor noise. In order to reduce the influence of noise, performing burst transmission of ultrasound (ultrasonic waves) multiple times and averaging the detection results is effective. However, if burst transmission is performed, an echo remains for a certain period of time, and accordingly, the next burst transmission cannot be performed until the echo has disappeared. Note that a time interval between the preceding burst transmission and the subsequent burst transmission will be called a burst interval. That is to say, the burst interval refers to the time from the timing at which the preceding burst transmission starts until the timing at which the subsequent burst transmission starts. If a long burst interval is set, the influence of an echo on the subsequent burst transmission is small. However, there may be cases where double-feed with a small amount of overlapping between the preceding sheet and the subsequent sheet (i.e., dragged double-feed) cannot be detected. This is because, the longer the burst interval is, the lower the number of times that double-feed detection can be executed while one sheet is conveyed is. A threshold value that is used for detecting double-feed is determined by performing burst transmission in a state where no sheet exists. When in a state where no sheet exists, an echo occurs over a long time, and therefore, the burst interval has to be long. On the other hand, in a state where a sheet exists, an echo occurs over a shorter time. Accordingly, if the burst interval used in a state where a sheet exists can be set shorter than the burst interval used in a state where no sheet exists, the number of times that burst transmission is performed for one sheet can be increased, and the accuracy of double-feed detection will improve. For example, dragged double-feed will also be able to be detected. The present invention enables more accurate detection of double-feed than with the conventional technique.